1. Field of the Invention
This invention relates to a process for removing relatively small quantities of sulfur dioxide gas from gaseous mixtures.
2. Brief Description of the Prior Art
In my U.S. Pat. No. 3,984,529, I review various prior efforts to provide economical and efficient processes for removing sulfur dioxide gas from gaseous mixtures, such as stack gases or the like. Removal of sulfur dioxide is desirable in order to obviate or reduce atmospheric pollution, and also in order to avoid the loss of the economic value represented by the recovered sulfur or sulfur compounds constituting potential end products of the recovery process.
In my referenced patent, I describe procedures and certain proposed methods which generally include the step of initially contacting the gaseous mixture which contains the sulfur dioxide with an aqueous solution containing a removal reactant so as to form a condensed phase in which the removed sulfur dioxide is combined chemically with the removal reactant in the form of sulfite or hydrosulfite compounds in aqueous solution. The removal reactants contemplated for use in the processes described include, inter alia, alkali metal hydroxides, and salts of weak acids and alkali metal hydroxides.
After formation of the pregnant scrubbing solution which contains compounds including combined sulfite ions derived from the extracted SO.sub.2 gas, the pregnant scrubbing solution is contacted with an organic liquid phase which includes, as an active component, certain nitrogen-containing water-immiscible organic compounds which have specified properties. Among such compounds which are effective and are preferred at this point in the process are various types of long chain alkyl amines which contain from about 12 to about 45 carbon atoms, and have a solubility in water of less than 0.2 gm/100 gms of water at 25.degree. C., and the water immiscible salts of these basic compounds. When the amine salts are used as the transfer reactants, their water solubility should not exceed about 5 gms/100 gms of water at 25.degree. C. Such amine salts are generally derivable from free amines having the degree of immiscibility prescribed above. The result of contacting the pregnant scrubbing solution with the described amine-containing organic liquid phase is to transfer the sulfite ions from the pregnant aqueous scrubbing solution to the organic phase as a result of chemical combination of sulfite ions with the long chain alkyl amines. A concomitant result of the described contact is the regeneration of the scrubbing solution containing the selected removal reactant so that it can be recycled to the scrubbing zone, and used for contacting the gaseous mixture from which additional SO.sub.2 is to be subsequently removed.
The organic phase developed upon contact of the long chain amine with the pregnant scrubbing solution contains long chain amine sulfites, and is contacted with hydrogen sulfide so as to reduce the sulfur-containing salts therein to elemental sulfur, and concurrently regenerate the long chain amine. The amine can then be reutilized for contacting additional sulfite-containing pregnant scrubbing solution from the scrubbing zone for the purpose of repeating the transfer of sulfite ions into the organic phase.
When an amine salt is used instead of an amine as the transfer agent, the transfer step will yield an acid in the aqueous phase. This acid can be removed by contacting this aqueous phase with the organic phase from the reduction step where the regenerated amine will combine with the acid to form the amine salt. After this "recontact", the aqueous solution can be recycled to the scrubber and the organic phase can be utilized as recycled transfer agent.
As described in my co-pending U.S. Application Ser. No. 728,174, an improvement on the foregoing described basic process comprises initially contacting the gas from which the sulfur dioxide is to be removed with an aqueous solution of an alkali metal sulfite. In the course of this scrubbing step, the SO.sub.2 in the gaseous mixture combines with the alkali metal sulfite to yield the hydrosulfite salt in aqueous solution. The pregnant aqueous solution resulting from scrubbing is next contacted with a liquid organic phase which contains one or more substantially water-immiscible long chain amine compounds capable of extracting sulfite ions from the pregnant scrubbing solution by chemical combination therewith. Upon the termination of intimate contact between the organic extractant phase and the aqueous phase, the alkali metal sulfite is regenerated in the aqueous phase so that the scrubbing solution thus formed can be recycled to the scrubbing zone of the process. The organic phase, which contains long chain amine sulfites upon completion of the transfer reaction, is then heated to a temperature sufficiently high that the amine sulfites therein undergo thermal decomposition to yield sulfur dioxide. The decomposition of the amine sulfites concurrently produces free amines which can be reused in carrying out further extraction of sulfite ions from additional scrubbing solution.
The sulfur dioxide generated by decomposition of the amine sulfite in the organic phase can be used as an end product, or it can be converted to elemental sulfur by various conventional methods known to the art.
In this process, high efficiency in the scrubbing step is achieved by using a regenerated scrubbing solution which has a high pH value. The pH value can be as high as 7. High efficiency in the extraction step is also achieved because the selectivity in this step of the process is as high as 100% (i.e., only sulfite ions are transferred into the organic phase.)